The present invention relates to a rotary wing aircraft rotor blade assembly, and more particularly to a rotary wing aircraft main rotor blade assembly with a high negative pitching moment airfoil section.
Conventional airplane propeller blade designs with aft loaded airfoils and high negative pitching moments are often utilized to enhance propeller performance. These airfoils generally have superior drag levels at design lift that increases propeller aerodynamic efficiency. The relatively large inherent airfoil pitching moments do not cause unacceptable torsional bending because propeller blades are rigid.
In general, aft loaded airfoils have not been used on rotary-wing aircraft main rotor blades to gain similar aerodynamic advantage since main rotor blades have higher aspect ratios and are relatively soft in torsion such that other favorable flight attributes may deteriorate. To maintain desired rotary wing attributes, the vast majority of rotary-wing aircraft utilize blade airfoils with quarter chord pitching moment between −0.02 and +0.02 prior to stall or transonic flow onset. In the few cases where this limit may be locally exceeded, airfoils with strong pitching moments in the opposite direction are used elsewhere on the rotor blade to mitigate adverse torsion bending. Negative attributes may include an increase in blade and control system weights, increases in aircraft vibration due to excessive blade torsion, and increases in high-speed rotor power and noise due to increased blade torsion. Blade and control system weights increase as equivalent fatigue life requirements must be maintained despite an increase in vibratory and steady loads. Affected components include the blade spar, blade control horns, control pushrods, the control swashplate scissors, and the swashplate activation servos. These components represent a significant portion of the total aircraft empty weight. Weight increases in these components directly impact aircraft mission payload capability. Furthermore, uncontrolled pitching moments from aft loaded airfoil may drive sufficient blade torsional bending to cause negative angles-of-attack and surface shocks on the advancing blade tip. The shocks increase required rotor power as well as the aircraft external noise signature.